This invention relates to an improved filter medium containing fibers which are configured to hinder the passage of particles.
Filter media having large interfiber pores and, thus, a high permeability typically contain sparsely packed relatively thick fibers. Such filter media require relatively low driving pressure to provide adequate filtration throughput rate and service life. However, highly permeable filter media, e.g., residential glass fiber HVAC filters, only provide a low filtration efficiency in that the large interfiber pore structures of the media do not have interstitial configurations that are suitable for entrapping fine contaminant particles. Consequently, coarse fiber, highly permeable, filter media have not been used in fine particle filtration applications.
In contrast, microfiber nonwoven webs, such as meltblown fiber webs, have been used as fine particle filter media. The densely packed fine fibers of these webs provide fine interfiber pore structures that are highly suitable for mechanically trapping or screening fine particles. However, the fine pore structure of meltblown fiber webs and other similar microfiber webs that have densely packed fine fibers results in a low permeability, creating a high pressure drop across the webs. Consequently, the low permeability of fine fiber filter media requires a high driving pressure to establish an adequate filtration throughput rate. Furthermore, as contaminants accumulate on the surface of the filter media, the contaminants quickly clog the small interfiber pores and further reduce the permeability of the media, thereby even further increasing the pressure drop across the media and rapidly shortening the service life.
Additionally, microfiber web filter media do not tend to have a physical integrity that is sufficient enough to be self-supporting. Although the physical integrity of microfiber filter media can be improved by increasing the basis weight or thickness of the web, the increased basis weight or thickness exacerbates the pressure drop across the filter media. As such, microfiber web filter media are typically laminated to a supporting layer or fitted in a rigid frame. However, the conventional supporting layer or rigid frame does not typically contribute to the filtration process and only increases the production cost of the filter media.
There remains a need for self-supporting filter media that provide combinations of desirable filter properties, including high filtration efficiency and particle retention, high permeability, low pressure drop, high throughput rate and long service life.
The present invention is directed to a lofted nonwoven filter medium which employs multilobal fibers to enhance the filtration efficiency and increase the particle holding characteristics of the web. Each of the multilobal fibers has a cross section defined by a plurality of raised portions separated by depressed portions. The multilobal fibers have the ability to catch, trap or ensnare particulate impurities in the depressed regions between the raised lobal regions, thereby effecting a high filtration efficiency without requiring the fibers to be small or closely packed together. The multilobal fibers can be used alone or in combination with fibers which are not multilobal, or which do not have depressed regions.
In one embodiment, the multilobal fibers are bicomponent, with a higher melting polymer portion and a lower melting polymer portion. The lower melting portion facilitates bonding of the fibers together, in conventional through-air and other bonding processes that can be used to form a coherent web filtration structure. The higher melting component helps maintain the multilobal structure of the fibers. The multilobal fibers can also be electret treated to facilitate attraction of dipolar particles.
With the foregoing in mind, it is a feature and advantage of the invention to provide an improved nonwoven web for filtration applications, which includes multilobal fibers having raised lobal portions and depressed portions.
It is also a feature and advantage of the invention to provide a filter medium including the improved nonwoven web.